This invention generally relates to operating a fuel cell power plant during cold temperature conditions. More particularly, this invention relates to selectively utilizing an enthalpy recovery device based upon operating conditions of a fuel cell power plant.
Fuel cell power plants are well known and used to produce electrical power from reducing and oxidizing fluids. Fuel cell structures and fuel cell power plant arrangements are known and come in a variety of configurations. Many fuel cells utilize a proton exchange membrane (PEM) as part of a chemical process for producing electrical energy.
In proton exchange membrane fuel cells, it is important that the reactants are humidified to prevent the PEM from drying out. If the PEM were to dry out, the cell resistance increases, which lowers the fuel cell performance. Additionally, if the membrane dries out, it degrades, which results in reduced service life for the fuel cell power plant.
Moreover, it is desirable that the fuel cell power plant operate within water balance. This means that the water removed from the power plant system as vapor in the reactant exhaust streams, or as liquid across the porous water transport plates, must be equal to or less than the amount of water formed as a result of reactions within the fuel cell. In one example power plant air utilization is approximately 60–70% with an operating pressure of about 15.7 PSIA. There is a relationship between system exhaust dewpoint and system air utilization that satisfies water balance, depending on the fuel provided to the power plant. In one example where gasoline is the fuel, an exhaust temperature of 110° F. provides adequate water balance. If the exhaust temperature exceeds 110° F., there is a water deficiency and the cells will dry out. If the exhaust temperature is less than 110° F., a system water surplus results, possibly flooding the cells unless the water is removed as liquid.
Enthalpy recovery devices (ERDs) are used in fuel cell power plants to exchange heat and humidity from a process exhaust stream to a reactant inlet stream. ERDs have been used to maintain adequate water balance within fuel cell power plants.
A particular challenge is presented when the ambient temperatures are low such that water or liquid associated with or processed by the ERDs may freeze. This is possible, for example, on a vehicle including a fuel cell power plant that is located in a region where winter temperatures may be at or below freezing. Under such conditions, the enthalpy recovery device may not function as required and the power plant system water balance may not be maintained, which could result in performance degradation or a reduced service life of the system.
There is a need for a fuel cell power plant system that is capable of operating in cold conditions. This invention addresses that need.